
The present invention relates to the removal of hydrogen sulfide from gas streams using a reaction medium comprising non-aqueous Lewis bases.
Many reservoirs of natural gas contain hydrogen sulfide and carbon dioxide which are acid gases which can be extremely corrosive when combined with each other and water. Natural gas containing such acid or sour gases must be purified (or xe2x80x9csweetenedxe2x80x9d ) to remove or decrease the concentration of such gases prior to the purified natural gas (xe2x80x9csweet gasxe2x80x9d ) being forwarded to consumer, industrial and other markets.
The most commonly-practised process technology for acid gas removal is the absorption of the acid gases from the natural gas stream by a regenerable absorbing solution in a gas processing plant. In such procedures, a regenerable absorbing solution is passed in countercurrent contact with the natural gas stream to absorb the H2S and CO2, as well as other sulfur compounds, from the natural gas stream, thereby reducing their concentration in the natural gas stream and purifying the natural gas stream.
The acid gas laden solution then is regenerated by steam stripping at elevated temperature and the regenerated solution is cooled and recirculated back to the natural gas contacting stage. Acid gases stripped from the solution in the regeneration step are vented from the gas processing plant for further processing, including, in some cases by incineration to sulfur dioxide.
Chemicals that are commonly employed in such procedures include amines, esters and similar regenerable materials in which the acid gases may be absorbed. The most commonly-employed amines for this procedure include monoethanolamine (MEA), diethanolamine (DEA) and methyldiethanolamine (MDEA).
The present invention is concerned with a manner of processing sour natural gas streams.
The present invention provides novel procedures for treatment of hydrogen sulfide-containing gas streams. As described in more detail below, the process of the invention includes a step of reacting hydrogen sulfide and sulfur dioxide to form sulfur (sometimes termed the Claus reaction) in a reaction medium comprising a non-aqueous Lewis base, preferably quinoline. The processes described herein are applicable to other gas streams which contain hydrogen sulfide, including Claus process tail gas streams and industrial flue gas streams. One benefit of the present invention, as described below, is that there is no necessity to precisely control the stoichiometry of the gases for a complete removal of hydrogen sulfide to be effected.
In accordance with the present invention, a reaction medium comprising a non-aqueous Lewis base, having a pKb value of about 6 to about 11, preferably about 8 to about 10, particularly quinoline, is used to remove hydrogen sulfide from gas streams, particularly in the sweetening of sour natural gas streams but also for other applications, such as tail gas cleanup, by effecting reaction between hydrogen sulfide and sulfur dioxide in the reaction medium. The reaction of hydrogen sulfide with sulfur dioxide, which may be in the form of a reaction product with the Lewis base, proceeds in accordance with the equation:
2H2S+SO2xe2x86x923S+2H2O
known as the Claus reaction.
It is well known that sulfur dioxide is soluble in many amines, including quinoline, forming an equi-molar solid reaction product, itself soluble in quinoline and quinoline-water mixtures. However, such material has a measurable sulfur dioxide vapor pressure which becomes significant at temperatures above about 50xc2x0 C. and hence is unsuitable as a storage medium for sulfur dioxide, at least for low pressure applications.
The inventors herein have discovered, surprisingly and unexpectedly, that non-aqueous Lewis bases, in the presence of water, hydrogen sulfide and sulfur product and over time form non-aqueous Lewis base miscible components which have a storage capacity for sulfur dioxide and which do not exhibit any significant sulfur dioxide vapor pressure. The storage capacity of the solution enables the reaction of sulfur dioxide and hydrogen sulfide to be carried out without precise control of the input stoichiometry, providing a xe2x80x9cfly-wheelxe2x80x9d effect. This aspect of the invention is particularly important for tail gas cleanup, such as of Claus reaction tail gas streams, where variations in gas composition commonly occur.
The exact nature of the miscible components which are formed in the non-aqueous Lewis base are unknown but are complex sulfur-oxide species, including sulfate and polythionate species, and the term xe2x80x9ccomplex sulfur-oxide speciesxe2x80x9d is used herein to denote such miscible components. However, their effect is to provide a medium uniquely suited to remove hydrogen sulfide from a gas stream containing hydrogen sulfide by reaction with sulfur dioxide.
The inventors herein use the complex sulfur-oxide species in an original manner to provide improved procedures for removing hydrogen sulfide from gas streams. The present invention uses a reaction medium comprising a Lewis base which has pKb values from about 6 to about 11, preferably about 8 to about 10. Although strong Lewis bases, (pKb less than about 6) tend to react irreversibly with sulfur dioxide, preventing the Claus reaction from occurring, weaker Lewis bases (pKb greater than about 11) do not appear to catalyze the Claus reaction. The Lewis bases of intermediate basicity (pKb from about 6 to about 11), as used herein, react reversibly with sulfur dioxide and catalyze the Claus reaction. Quinoline (pKb 9) is the preferred Lewis base but other amines with the required pKb values can be used, such as 2,4,6-trimethyl pyridine (pKb 7). In addition, the Lewis bases may be used in diluted form with miscible liquids, such as glycols and N-methyl pyrrolidone (pKb 8 to 10).
One aspect of the present invention is directed specifically to scavenging hydrogen sulfide from gas streams, such as sour gases, on an intermittent or continuous basis, using a reaction medium which is a non-aqueous Lewis base xe2x80x9cpre-loadedxe2x80x9d with sulfur dioxide, but yet exhibits no significant sulfur dioxide vapor pressure, thereby providing a reservoir of reactant for the hydrogen sulfide. By contacting a gas stream containing hydrogen sulfide with the reaction medium, the hydrogen sulfide is absorbed and reacts with the pre-loaded sulfur dioxide at ambient temperature, about 50 to about 35xc2x0 C., to produce solid sulfur and water, according to the Claus reaction.
In this procedure, the pre-loaded sulfur dioxide is not stripped from the reaction medium and yet is available for reaction with the hydrogen sulfide, enabling removal of hydrogen sulfide from sour gas down to below about 4 ppm in one pass through the reactor.
The pre-loaded sulfur dioxide is present in the reaction medium in the form of complex sulfur-oxide species, including sulfate and polythionate species, and it is those species which react with the hydrogen sulfide absorbed from the gas stream.
Accordingly, in one aspect of the present invention, there is provided a reaction medium for removing hydrogen sulfide from a gas stream, which comprises a non-aqueous Lewis base having a pKb value of about 6 to about 11 containing complex sulfur-oxide species, including sulfate and polythionate species and exhibiting no sulfur dioxide vapor pressure and having the capacity to absorb hydrogen sulfide form a gas stream and to react with absorbed hydrogen sulfide to produce sulfur and water.
The reaction medium which is provided in accordance with this aspect of the present invention is formed by reacting hydrogen sulfide with sulfur dioxide to form sulfur and water. A stoichiometric excess of sulfur dioxide reacts with the in-situ produced sulfur and water to form the complex sulfur-oxide species. This operation may be effected in two steps or a single step. The gas stream may contain hydrogen sulfide and sulfur dioxide in a mole ratio of 1:1 to about 1.2.
Accordingly, in a further aspect of the present invention, there is provided a process for forming a reaction medium useful for the removal of hydrogen sulfide from a gas stream, which comprises reacting hydrogen sulfide and sulfide dioxide in a non-aqueous Lewis base having a pKb value of about 6 to about 11 to form sulfur and water in accordance with the equation:
2H2S+SO2xe2x86x923S+2H2O
and, either subsequently or simultaneously, reacting sulfur dioxide in excess of the stoichiometry of the equation with said non-aqueous Lewis base, to provide a reaction medium containing complex sulfur-oxide species, including sulfate and polythionates.
The complex sulfur-oxide species also may be formed by adding oxygen to the medium instead of sulfur dioxide, under appropriate conditions of temperature and pressure.
In the case where the reaction with the stoichiometric excess of sulfur dioxide is effected subsequent to completion of the reaction of hydrogen sulfide and sulfur dioxide, the process may be effected by:
feeding a gas stream containing said hydrogen sulfide and sulfur dioxide through a body of said non-aqueous Lewis base to form sulfur and water in accordance with said stoichiometric equation until there is a significant and sufficient amount of elemental sulfur formed, and, subsequently
feeding a gas stream containing sulfur dioxide to said body of Lewis base until a predetermined amount of sulfur dioxide has been added or there is a significant detectable amount of sulfur dioxide in the off-gas stream from the body of non-aqueous Lewis base.
In the case where the reaction with the stoichiometric excess of sulfur dioxide is effected simultaneously with the reaction of hydrogen sulfide and sulfur dioxide, the process may be effected by:
feeding a gas stream containing hydrogen sulfide and a stoichiometric excess of sulfur dioxide through a body of non-aqueous Lewis base until a predetermined amount of sulfur dioxide has been added or there is a significant detectable amount of sulfur dioxide in the off-gas stream from the body of non-aqueous Lewis base.
The novel reaction medium provided herein may be used to effect the removal of hydrogen sulfide from gas streams by absorption of the hydrogen sulfide from the gas stream and reaction of the absorbed hydrogen sulfide with the complex sulfur-oxide species. Based on the experimental results provided herein, a 4 ft. diameter gas-liquid contact column 20 ft. high, containing 1600 gallons of the reaction medium, is able to remove 1300 kg of hydrogen sulfide from sour gas before it is exhausted.
When the reaction medium is exhausted of the capacity to absorb hydrogen sulfide, the spent reaction medium may be regenerated in any convenient manner, for example, by reaction with sulfur dioxide or oxygen.
The reaction medium provided herein, pre-loaded with sulfur dioxide, has considerable benefits in hydrogen sulfide scavenging operations. Inexpensive sulfur dioxide, in the form of the complex sulfur-oxide species, is used as the scavenging agent, and the reaction medium has a high capacity for hydrogen sulfide removal, with up to 20 weight percent loadings of sulfur dioxide in the reaction medium.
The hydrogen sulfide removal process using the reaction medium may utilize a simple bubble column reactor and an ambient operating temperature, with minimal peripheral equipment, and have low capital costs. The simplicity of the hydrogen sulfide removal process makes it suitable for unattended wellhead operation, even in remote locations.
The reaction medium is readily loaded and unloaded from a bubble column and the potential exists for execution of the two operations simultaneously without a shutdown. The spent reaction medium is readily regenerated, which may be effected at a central processing facility, by adding more sulfur dioxide, as mentioned above. Such a central processing facility may serve a number of satellite wellhead sweetening operations, with recharged reaction medium being transported from the facility out to the sites and spent reaction medium being transported back to the central facility for regeneration. Since the sulfur dioxide is present in the reaction medium as complex sulfur-oxide species, the sulfur dioxide is not removed from the reaction medium by gas passing through the reaction medium and hence does not contaminate the sweetened gas.
Another aspect of the present invention provides a process for the removal of hydrogen sulfide from a gas stream by reaction with sulfur dioxide, which comprises effecting the reaction in a reaction medium comprising a non-aqueous Lewis base with a pKb value in the range of about 6 to about 11 and which reaction medium:
a) absorbs sulfur dioxide and reacts chemically therewith, the reaction medium containing water, hydrogen sulfide and sulfur to form complex sulfur-oxide species, including sulfate and polythionate species, whereby the resulting solution exhibits no or an insignificant sulfur dioxide vapor pressure;
b) absorbs hydrogen sulfide;
c) removes the hydrogen sulfide from the gas stream through contact of the gas stream with the reaction medium in the presence of the complex sulfur-oxide species;
d) acts as a catalyst for the overall reaction of the hydrogen sulfide with sulfur dioxide to produce sulfur; and
(e) has the capacity to absorb sulfur dioxide in sufficient quantity to remove substantially all the hydrogen sulfide from the gas stream, notwithstanding short term variations in the stoichiometric balance between the hydrogen sulfide and the sulfur dioxide in the reaction medium.
In accordance with the aspect of the invention described above, the reaction medium may be preloaded with sulfur dioxide, whereby the complex sulfur-oxide species are pre-formed in the reaction medium before contacting with the gas stream.
The reaction medium may consist essentially of the non-aqueous Lewis base or may further comprise a miscible diluent, including glycols, water and N-methyl-pyrrolidone.
The process of hydrogen sulfide removal provided herein may be effected in a manner in which sulfur dioxide is continuously absorbed by the reaction medium to react with hydrogen sulfide in the gas stream at a temperature of about 5xc2x0 to about 155xc2x0 C., preferably about 30xc2x0 to about 130xc2x0 C., to produce sulfur.
The sulfur which is formed in the process is completely miscible with the preferred Lewis base, quinoline, at temperatures above about 80xc2x0 C. Removal and recovery of the sulfur may be achieved by operating a portion or the whole of the hydrogen sulfide removal operation at a temperature below this temperature. The recovered sulfur may be washed with hot water or suitable solvent, or subjected to an alternative procedure, to remove Lewis base.
The latter procedure is particularly useful in a natural gas sweetening operation or for the processing of a hydrogen sulfide-containing gas stream where a continuous operation is required.
The process of hydrogen sulfide removal provided herein may be effected in a manner in which the gas stream is so contacted, intermittently or continuously, with a body of the reaction medium to react the hydrogen sulfide with sulfur dioxide in the reaction medium in the presence of complex sulfur-oxide species to form sulfur until the reaction medium is depleted of its capacity to react with hydrogen sulfide and a significant hydrogen sulfide vapor pressure is present.
The latter procedure is particularly useful for scavenging operations to remove lesser amounts of hydrogen sulfide on an intermittent operational basis from gas streams having a variety of sources. The procedure may be operated at a temperature above or below the melting point of sulfur and down to the solidification point of the reaction medium. The sulfur usually is permitted to accumulate in the body of the reaction medium-until the reaction medium is depleted.
When the reaction medium becomes depleted of the ability to react with hydrogen sulfide, which may be detected by any conventional sensing device, the reaction medium is regenerated. Regeneration may take a variety of forms, including replacement of the depleted reaction medium by a fresh charge of the reaction medium containing complex sulfur-oxide species or a charge of reaction medium containing complex sulfur-oxide species regenerated from a previous batch. Sulfur may be removed intermittently as desired from the reaction medium using conventional technology.
By means of variations in the composition of the reaction medium, to include suitable amounts of glycol and water, and by the choice of a suitable temperature of operation, it may be possible to combine the removal of hydrogen sulfide with the simultaneous removal of carbon dioxide and water vapour from the gas stream.